THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FORM MODERN FRESHWATER SETTINGS1

Some modern laminated find calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolites growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventually become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a propertuy they share with cyanobacterial stromatolite builders.     

THE ECOLOGICAL IMPLICATIONS OF GROWTH FORMS IN EPIBENTHIC DIATOMS 1

This paper evaluates the utilisation of space by epibenthic diatom cells, as a response to environmental variations. The aggregation pattern of five species of epibenthic diatoms was quantified and compared to provide evidence for the significance of cell motility as an adaptive mechanism for space occupation and monopoly. The epibenthic diatoms included (1) non-mobile colonial species forming either fan-shaped (Synedra tabulata (Ag)Kz.) or arborescent (Gomphonema kamtschaticum var. californicum Grun.) colonies; (2) slow-moving (Cocconeis costata Greg, and Amphora pusio Cl.), and (3) fast-moving (Navicula direct a (W. Sm.) Ra.) non-colonial species. The aggregation pattern of S. tabulata did not vary significantly among six different light intensities manipulated in nature. The major patterns of aggregation were identified using analysis of covariance and dummy-variable regression. Highly mobile N. directa are significantly less aggregated than the four other diatom species. Non-mobile and slow-moving species show a similar, highly aggregated pattern. The occurrence of two patterns of spatial dispersion indicates that growth forms bear far-ranging ecological implications with respect to colonization strategies, immigration, and possibly impact by grazers. An integrated model of growth form characteristics, biological properties, and ecological implications is presented for epibenthic diatoms.     

THE EFFECT OF IRRADIANCE ON THE GROWTH OF DIFFERENT FORMS OF FRESHWATER DIATOMS: IMPLICATIONS FOR SUCCESSION IN ATTACHED DIATOM COMMUNITIES

The growth rates of 12 strains of attached freshwater diatoms were examined as a function of irradiance. The results were used to determine physiological parameters in the growth–irradiance relationships. Values of G _max (maximum growth rate) and I  _k (irradiance at half of G _max) correlated with successional phase. The development of attached diatom communities appears to proceed from high irradiance species to low irradiance species that appear later in the successional sequence. Thus, efficiency of light use is linked to successional strategy, as has been documented for terrestrial plants.     

FRESHWATER DIATOMS FROM THE CANADIAN ARCTIC TREELINE AND DEVELOPMENT OF PALEOLIMNOLOGICAL INFERENCE MODELS1

Relationships between surface sediment diatom assemblages and measured environmental variables from 77 lakes in the central Canadian arctic treeline region were examined using multivariate statistical methods. Lakes were distributed across the arctic treeline from boreal forest to arctic tundra ecozones, along steep climatic and environmental gradients. Forward selection in canonical correspondence analysis determined that dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), total nitrogen (TN), lake surface area, silica, lake‐water depth, and iron explained significant portions of diatom species variation. Weighted‐averaging (WA) regression and calibration techniques were used to develop inference models for DIC, DOC, and TN from the estimated optima of the diatom taxa to these environmental variables. Simple WA models with classical deshrinking produced models with the strongest predictive abilities for all three variables based on the bootstrapped root mean squared errors of prediction (RMSEP). WA partial least squares showed little improvement over the simpler WA models as judged by the jackknifed RMSEP. These models suggest that it is possible to infer trends in DIC, DOC, and TN from fossil diatom assemblages from suitably chosen lakes in the central Canadian arctic treeline region.     

A PRELIMINARY INVESTIGATION OF THE PHYLOGENETIC RELATIONSHIPS AMONG THE FRESHWATER,APICAL PORE FIELD-BEARING CYMBELLOID AND GOMPHONEMOID DIATOMS (BACILLARIOPHYCEAE)1

Relationships among the apical pore field-bearing diatom genera Brebissonia, Cymbella, Gomphonema, Gomphoneis, Didymosphenia, Gomphocymbella, and Reimeria and their close ally Encyonema were evaluated using cladistics. These biraphid genera are a polythetic group defined by chloroplast number and type, mode of sexual reproduction, valve orientation and presence of apical pore fields. Character states of valve as well as non-siliceous features were described and then polarized using the outgroup method, with the naviculoid genera Anomoeoneis and Placoneis serving as outgroups. The cladistic analysis suggests two groups of terminal taxa, corresponding to to a predominantly cymbelloid lineage and a gomphonemoid lineage. Didymosphenia, previously thought to be closely allied to gomphonemoid diatoms, is shown to be more closely related to Cymbella. Gomphonema appears to be without distinguishing characteristics. Relationships of the other genera are described. The systematic position of small, doubly-punctate Gomphonema species is discussed.     

DIATOMS (BACILLARIOPHYTA) OF ISOLATED ISLANDS: NEW TAXA IN THE GENUS NAVICULA SENSU STRICTO FROM THE GALáPAGOS ISLANDS1

The diatoms (Bacillariophyta) from a coastal lagoon from the Diablas wetlands (Isla Isabela, the Galápagos Islands) were studied in material from surface samples and a sediment core spanning the past 2,700 years in order to examine evidence of diatom evolution under geographic isolation. The total number of taxa found was ～100. Ultrastructural variation in valve morphology between members of Galápagos taxa was used to describe 10 species from the genus Navicula sensu stricto, which are new to science. Four taxa: N. isabelensis, N. isabelensoides, N. isabelensiformis, and N. isabelensiminor, shared several key characteristics that may be indicative of a common evolutionary heritage; these species therefore provide possible evidence for the in situ evolution of diatoms in the Galápagos coastal lagoons. Shared morphological characteristics include: (i) stria patterning in the central area, (ii) an elevated and thickened external raphe‐sternum, (iii) external central raphe endings that are slightly deflected toward the valve primary side, and (iv) an arched valve surface. To explain these findings, two models were proposed. The first suggested limited lateral diatomaceous transport of Navicula species between the Galápagos and continental South America. Alternatively, these new species may be ecological specialists arising from the unique environmental conditions of the Galápagos coastal lagoons, which restrict the colonization of common diatom taxa and enable the establishment of novel, rare species. The Diablas wetlands are an important site for diatom research, where local‐scale environmental changes have combined with global‐scale biogeographic processes resulting in unique diatom assemblages.     

EFFECTS OF LIGHT AND SUBSTRATE ON THE BENTHIC DIATOMS IN AN OLIGOTROPHIC LAKE: A COMPARISON BETWEEN NATURAL AND ARTIFICIAL SUBSTRATES1

Benthic diatoms form a particularly important community in oligotrophic lakes, but factors influencing their distribution are not well known. This study reports the depth distribution of living motile and total diatoms (living plus dead diatoms) on both natural (from sand to fine organic mud) and artificial substrates in an oligotrophic lake. On artificial substrates, motile diatom densities peaked in abundance (24–30 cells · mm^?2) between 0.6 and 1.9 m depth; on natural sediment surfaces, motile diatoms were generally more numerous and peaked in abundance (925 cells · mm^?2) at 1.3 m depth. Total diatom densities on artificial substrates were highest (1260 valves · mm^?2) at 0.6 m depth, with very low values below 3 m depth; on natural sediment surfaces, total diatom abundances were generally much higher (21600 valves · mm^?2) at 3 m depth and declined gradually with depth. Significant relationships were found between light and diatom densities on the artificial substrate. Ordination analysis indicated that substrate type significantly correlated with the variation of diatom composition on artificial and natural substrates. Our results suggest that in oligotrophic lakes, light influences benthic diatom abundance, whereas substrate type has more influence on benthic diatom composition.     

INFERRING TAXONOMIC POSITIONS AND TESTING GENUS LEVEL ASSIGNMENTS IN COCCOID GREEN LICHEN ALGAE: A PHYLOGENETIC ANALYSIS OF 18S RIBOSOMAL RNA SEQUENCES FROM DICTYOCHLOROPSIS RETICULATA AND FROM MEMBERS OF THE GENUS MYRMECIA (CHLOROPHYTA, TREBOUXIOPHYCEAE CL. NOV.)1

Complete nuclear-encoded small-subunit ribosomal RNA (18S rRNA) coding sequences were determined for the coccoid green algae Dictyochloropsis reticulata (Tschermak-Woess) Tschermak-Woess , Myrmecia astigmatica Vinatzer, and M. bisecta Reisigl, to investigate the taxonomic position of Dictyochloropsis Geitler and of the genus Myrmecia Printz. Phylogenies inferred from these data revealed a sister-group relationship between D. reticulata and certain coccoid green algae that lack motile stages (autosporic coccoids) within the order Microthamniales. The monophyletic origin of the Microthamniales, including autosporic coccoids previously classified in the Chlorophyceae, is clearly resolved by the rRNA sequence data. This finding. shows the considerable taxonomic breadth of that order, whose taxonomic position has been unclear so far. A new class, Trebouxiophyceae, is proposed for this group of green algae. Phylogenetic inferences from the rRNA sequences show paraphyly of the genus Myrmecia. The 18S rRNA sequence data suggest that, among taxa that share similar vegetative cell morphologies, the zoospore characters resolve better the actual genus and species boundaries. Within identical zoospore types, the rRNA data allow further resolution of taxonomic relationships. On the basis of the.se findings, I propose that the genus Friedmannia Chantanachat ± Bold be merged into Myrmecia and that only those species be left in the genus Myrmecia that are identical in particular zoospore characters (i.e. those described in detail for M. israeliensis ( Chantanachat ± Bold) comb, nov.), namely M. astigmatica, M. biatorellae (Tschermak-Woess ± Ptesst) Petersen, and M. israeliensis. Myrmecia bisecta has to be excluded from Myrmecia; its taxonomic position within the Trebouxiophyceae is unclear .